The present invention relates to an expanding mandrel, that is to say to a mandrel designed to grip a workpiece to be machined (milling, turning, grinding) by pressing an expandable portion of the mandrel against the walls of an axial bore provided in said workpiece. Expanding mandrels are usually secured to the forward face of a machine spindle using a locking nut.
There is also known a first type of expanding mandrel, the body (xe2x80x9cmandrel shaftxe2x80x9d) of which comprises a forward portion having, prior to gripping, the shape of a hollow cylinder provided with several longitudinal slits. This split portion is caused to expand by sliding a conical head rod along its axis, so that the split portion assumes, during gripping, a conical shape the apex angle of which progressively increases. When said mandrel is mounted on the machine spindle, the rear portion of the conical head rod is screwed onto a tie member which can be caused to slide along the axis of the machine spindle to control the gripping or releasing of the workpiece.
This type of mandrel is relatively simple to manufacture, and inexpensive, but it has a number of drawbacks. Firstly, the expansion of the split portion of the mandrel shaft is conical, while the conical head rod is not guided by the shaft: consequently, the expansion of the split portion is irregular, with the result that gripping lacks precision. Secondly, the split portion of the shaft lacks rigidity and is liable to be deformed, or even fail, when the gripping mechanism is actuated in the presence or, which is worse, in the absence, of a workpiece; in this case, the only way of replacing the defective part is to replace the entire mandrel. Finally, the gripping diameter characterising each mandrel is unique, to within one finishing operation, with the result that the user has to change mandrels when he wishes to machine a workpiece having a bore with a diameter that differs sufficiently from that of the previous workpiece. Such drawbacks are obviously time consuming and lead to considerable extra cost.
In a second type of conventional mandrel (manufactured, moreover by the Applicant of present invention), the shaft has a forward pointing forward part of a conical shape. Unlike the mandrel described above, therefore, it is the expanding portion in this case that will slide axially to effect gripping or releasing. This expanding portion is constituted by a ring provided with longitudinal slits (xe2x80x9cthe sleevexe2x80x9d), which has, at least on its rear portion, an inner wall having the shape of a cone, or taper, the apex angle of which is equal to that of the cone over which the sleeve slides. The outer surface of the sleeve thus remains rigorously cylindrical during its expansion, thus ensuring perfectly precise, uniform gripping. In addition, there is no risk of failure or plastic deformation of the sleeve when it is caused to slide, in the presence or in the absence of a workpiece. Finally, to change over from one workpiece to another having a different bore diameter, it suffices to replace the sleeve (together with, if applicable, the portion of the drive mechanism attached to the sleeve), and not the entire mandrel.
A distinction can be made, moreover, generally speaking, in connection with this second type of conventional mandrel, between two types of drive mechanism, depending on whether the sleeve is driven in the area of its forward portion or of its rear portion. Devices for driving from the rear are quite suitable for gripping diameters that are relatively small (less than 15 mm, for instance), but they are all excessively bulky in the case of large gripping diameters as they are attached to the sleeve on its periphery. Devices for driving from the front use a piston with is precisely guided in a cylindrical bore provided along the axis of the mandrel shaft, from which it projects forwards, the forward ends of said piston and of the sleeve being fixed to one another. These forward drive devices provide the sleeve with strong internal support over the rear portion of the sleeve (since this rear portion bears on the conical portion of the shaft), but, in known designs, they do not offer strong internal support over the entire length of the sleeve; this (relative) fragility can prove particularly inconvient when the sleeve is placed in contact with the workpiece to be gripped only over the forward portion of the sleeve, as one is sometimes compelled to do when the bore of the workpiece to be gripped is not very deep (xe2x80x9cshort grippingxe2x80x9d).
The object of the present invention is thus to provide an expanding mandrel ensuring gripping that is both precise and strongly supported, whatever the gripping length.
Another object of the present invention is to provide an expanding mandrel that is not bulky, whatever the gripping length.
Yet another object of the present invention is to provide an expanding mandrel permitting quick sleeve change, in such a way that it is possible, using the same mandrel body, to change over in a short space of time from one workpiece to be gripped to another having a different bore diameter.
These objects are achieved by means of a expanding mandrel comprising a shaft the forward end of which has the shape of a forward pointing truncated cone, an expanding sleeve constituted by a ring provided with longitudinal slits designed to slide over said conical forward end, and a piston which is guided in a cylindrical bore provided along the axis of the mandrel shaft, from which it projects forwards, with the sliding of said piston driving said sleeve so as to control the gripping or the releasing on said mandrel of a workpiece. According to the invention, the forward end of the piston has the shape of a rearward pointing truncated cone, and the inner wall of the sleeve has the shape of two successive truncated cones orientated head to tail (i.e. in mirror position), with the apex angle of the forward cone of the sleeve, which is capable of mating with said cone of the piston, being greater than the apex angle of the rear cone of the sleeve, which is capable of mating with said cone of the shaft.
According to certain particular characteristics, the cylindrical portion and the frustoconical portion of the piston are separated by a member capable of co-operating with a member separating the two conical portions of the inner wall of the sleeve.
According to certain additional characteristics, the piston is capable of being screwed at the rear, by inserting a screwdriver into a corresponding frontal recess provided on the piston, onto a pulling member sliding axially in said bore of the shaft.
According to certain particular characteristics, in said pulling member there has been transversely pierced a threaded through bore into which can be introduced a stop screw passing either through an oblong hole, or through a hole having a diameter corresponding to said stop screw, the positions of these two holes being in the rear portion of the shaft at diametrically opposed points.